In one modern cellular communications system a mobile station or terminal bidirectionally communicates with a Base Station (BS), Mobile Switching Center (MSC) and Interworking Function, referred to collectively as the BMI. In this system, referred to in the art as IS-136, it is possible for the mobile terminal to use digital traffic and control channels which are organized in a time division duplex (TDM) format with a slotted frame structure. The slotted frames are further organized into a logical structure comprised of repeating Superframes and Hyperframes.
FIG. 4 illustrates a conventional TDM frame format that is used by this system in the forward direction, i.e., from the BMI to the mobile terminal. One frame is composed of six slots, each of which is partitioned into a plurality of fields. Of most interest to this invention are the 12-bit Coded Digital Verification Code (CDVCC) and the 11-bit Coded Digital Control Channel Locator (CDL) fields that are transmitted in a digital traffic channel.
The content of the CDVCC field is defined in IS-136.2, Rev. A, Section 1.2.5, as being a 12-bit field permitting 255 distinct values of CDVCC. The DVCC is an 8-bit word which is coded using a (15,11) Hamming code shortened to form the 12-bit CDVCC.
The content of the CDL field is defined in IS-136.2, Rev. A, Section 1.2.6, as containing a coded version of Digital Control Channel (DL) values. The CDL provides information that may be used by the mobile terminal to assist in the location of a Digital Control Channel (DCCH). For cellular frequencies, a properly decoded DL value indicates that a DCCH may be found on an RF channel number in the range ((8*DL)+1) to ((8*DL)+8), provided the RF channel number is valid. For PCS frequencies, the DL to channel number mapping is given by ((16*DL)+1) to ((16*DL)+16).
The channel encoding of the DL into CDL is said to be similar to that of the CDVCC. That is, the d7 bit is omitted (set to zero) in the encoding process and is not transmitted. Seven bits of the CDL are thus data bits and convey the DCCH RF channel information, while the remaining four bits are used for performing a cyclic redundancy check (CRC) calculation.
When a mobile terminal is powered on it begins to search for a control channel, either digital or analog (the digital control channel (DCCH) is preferred). The purpose of the CDL is to aid the mobile terminal in locating a DCCH.
However, due to RF channel impairments, such as fading and the like, and due to the relatively short length of the CDL field, it is possible that the 11 bits containing the CDL are correctly decoded and the CRC calculation indicates the presence of valid data, even when the data is incorrectly received. In this case the mobile terminal will attempt to tune to the erroneously indicated RF channel to locate and synchronize to a non-existent DCCH. This type of failure can significantly increase the amount of time that the mobile terminal must spend before it finally comes to the system serving state where it is able to originate and/or receive calls and messages.